Sign-in/Sign-out is not working properly

I am using Firefox 3.5.1. For two days I've tried to sign-in to Twitter and when I entered my username/password and hit Sign in, my info disappears and nothing happens. No login, no page change. So I close the tab and re-open it and go to Twitter and I am logged in. So then I click sign-out and nothing happens. It doesn't sign me out, the page just sits there. So I hit it a second time and it logs me out. Then I try logging in again and the pattern continues. I have added Twitter.com as an exception in my Cookies, but this did not resolve the issue. Is it possible this has something to do with your new front page? Please advise on why this is happening.